1 . Field of the Invention
This invention relates to a system for detecting an operational failure of a rack and pinion type of hydraulically controlled power steering system installed in a motor vehicle, and, more particularly, to an operational failure detecting system for detecting an operational failure of a device which detects an operating condition of a rack and pinion type of hydraulically controlled power steering system on a basis of a rise in hydraulic pressure and which is utilized to assist the driver in manipulating front wheels of the vehicle.
2 . Description of the Related Art
Hydraulically controlled power steering systems have been well known and widely installed to motor vehicles. Such a hydraulically controlled power steering system employs a power cylinder and a control valve provides a power assist. The power cylinder has a piston formed as an integral part of a rack of a rack and pinion steering gear, and the control valve is disposed in a hydraulic control circuit between the power cylinder and a power steering pump driven directly by an engine. Turning of a steering wheel, either the right or left, actuates the control valve to transmit and increase hydraulic oil pressure in one of right and left turn power chambers or the other of the power cylinder, so as thereby to reduce the driver's wheel turning effort, and hence to assist the driver in manipulating the steering the front wheels. An example of such a hydraulically controlled power steering systems can be found in Japanese Unexamined Patent Publication No. 4-59483.
It has been well known to those skilled in the automobile art to utilize engine speed feedback control such that the engine attains a desired or target speed of rotation during idling. Typically, when the engine is to drive various pieces of equipment as engine loads, such as the power steering pump for generating hydraulic pressure for the power assist, an alternator, a compressor for an air conditioner and the like, while running idle, the engine speed is increased with the effect of decreasing the effect of the load on the engine, avoiding a halt of the engine and increasing efficiency of the equipment. For this reason, in cases where a self-diagnosis is executed to detect whether the idle speed control is properly performed, a pressure sensor or a pressure switch is used to detect an increase in hydraulic oil pressure to a specified level prescribed for a determination on the operation of the hydraulically controlled power steering system and avoid execution of the diagnosis of failure of the idle speed control system while the hydraulically controlled power steering system is working as a load on the engine.
The pressure switch is comprised of, for instance, a pressure level plate and an elastically deformable member, such as a coil spring, for urging the pressure level plate against the pressure head. When the pressure level plate is forced against the spring by a specified level of pressure, it provide an electric signal in response to development of the specified level of pressure. Another type of pressure switch employs a diaphragm which is elastically deformable to provide an electric signal in response to development of the specified level of pressure.
In these pressure switches, operational failures occur due to entrapment of the spring or the diaphragm, or a lock of the pressure level plate caused by foreign particles, which always results in inaccurate detection of the specified level of pressure. For example, even though the hydraulically controlled power steering system is at rest without a rise in pressure level, it is possible to conclude that the hydraulically controlled power steering system is working, which results from wrong detection of the pressure level. In such cases, rest of the hydraulically controlled power steering system, which is one of the condition of execution of the diagnosis of operational failure of the idle speed control system, is not detected for a long time, resulting in interruption of the diagnosis of operational failure of the idle speed control system.